1. Field of the Invention
This invention relates to an exhaust gas cleaning apparatus which is generally provided at a midpoint of an exhaust system of an automobile and which is composed of a metal-made honeycomb core body adapted for carrying thereon an exhaust gas cleaning catalyst and fitted in a tubular metal casing. More particularly, the invention relates to an exhaust gas cleaning apparatus in which the constituent members of the apparatus are joined with adequate strength so as to resist against separation, cracking and other breakage due to thermal strains.
2. Description of the Related Art
In conventional exhaust gas cleaning apparatuses of the mentioned type, a sheet-like band made of a heat-resistant thin metal sheet and a corrugated band made from another thin metal sheet of the same kind are superposed one over the other to have areas of contact therebetween. They are then rolled together spirally into a honeycomb form (hereinafter called "honeycomb core body") defining many network-patterned gas flow passages along the central axis thereof for allowing exhaust gas to pass therethrough. The honeycomb core body is enclosed by a tubular metal casing which has a single-layer body and opens at opposite ends thereof. The honeycomb core body and the tubular metal casing are firmly joined by brazing, for example, so as to resist against the stress due to thermal strains resulting from the high temperature of exhaust gas as well as the exothermic reaction between exhaust gas and the exhaust gas cleaning catalyst, and also so as to resist against the severe vibrations while the automobile is running. The members of the honeycomb core body, i.e., the sheet-like band and the corrugated band are joined together at the areas of contact therebetween by one of various known manners.
However, this conventional arrangement cannot remain useful for a long period of time. For example, large separation between the outermost peripheral surface of the honeycomb core body and the inner surface of the metal casing tends to occur, in the direction of the axis of the honeycomb core body (i.e., in the exhaust gas flowing direction), due to the stress resulting from thermal strains under the above-mentioned severe thermal environments. Further, about the peripheral part (the outer peripheral surface and the portion immediately thereinside) of the honeycomb core body, the members (a sheet-like band and a corrugated band) of the honeycomb core body tend to be separated, cracked or otherwise broken to a large extent.
Attempts have been made to solve the foregoing problem, and it has turned out that to merely join the honeycomb core body and the metal casing together firmly is not successful.